Sensing arrangements are known which permit automatic alignment between the forks of a material handling vehicle, for example, an automatic guided vehicle of the driverless type, and a load to be lifted. Some examples of sensing arrangements of the optical type are shown in U.S. Pat. No. 3,672,470 dated June 27, 1972 to Frederick F. Ohntrup et al., and U.S. Pat. No. 4,279,328 to Sten H. N. Ahlbom dated July 21, 1981. Each of these patents utilize an optical system which enables the forks of the lift mast assembly to be elevationally positioned and aligned relative to a load to be lifted when the forks are empty. However, when the forks have a load supported thereon and the vehicle is in the process of unloading or stacking, the load carried by the forks does not enable automatic alignment between the stack upon which the load is to be deposited and the load itself. The optical sensing systems disclosed in the above-noted patents are intended for controlling the elevational position of the forks relative to the load to be lifted and not the position of the forks relative to the stack upon which it is to be placed. It would be inappropriate to utilize any of the above-noted sensing systems for load stacking since any load carried on the forks would interfere with delivery and receipt of the light signal. The position of the optical sensors shown in the above-identified patents is set to align the tip of the fork relative to the load to be engaged and therefore would not be properly positioned to identify the top of stack upon which the load is to be deposited.
U.S. Pat. No. 3,672,470 discloses the placement of an optical sensor at a location on and adjacent the tip of the lift fork. Often the environment in which the material handling vehicle operates is dirty, dusty and the like. This results over time in a build up of dirt on the optical sensors which adversely affects the reliability of the optical sensing system. This is particularly true of sensors located adjacent the tip of the load handling fork.
Because the optical sensors are mounted at exposed locations on the vehicle, the potential for damage caused by impact between the sensors and external objects and the like is considerable. The optical sensor disclosed in U.S. Pat. No. 3,672,470 is rigidly connected to the lift fork at a location closely adjacent the lift fork tip. In a normal loading operation, engagement between the fork tip and the load to be lifted is a common occurrence. Thus, the potential for damage to the optical sensor is clearly a strong possibility.
The optical sensor disclosed in the U.S. Pat. No. 4,279,328 is mounted on the carriage of the lift mast and elevationally movable relative the forks in response to the carriage being moved to ground level for lifting a load that is at rest on the ground surface. Since the optical sensor is not controllably elevationally movable independently of the position of the carriage, the optical sensor is below the forks at a lowered position during a major portion of a normal work cycle in order to be able to direct a signal and receive a reflection of the signal for fork alignment purposes with a load to be lifted. Therefore, the optical sensor is normally exposed and vulnerable to external forces which may cause damage to the optical sensors and vehicle down time. No provision is made to prevent excessive external forces from being applied to the optical sensor when the optical sensor is at the lowered operative position.
The present invention is directed to overcoming one or more of the problems as set forth above.